1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle. More specifically, the invention relates to an anti-skid brake control system which assures recovery of wheel speed even on a low friction road.
2. Description of the Background Art
As is well known, anti-skid brake control is performed generally for maintaining vehicular wheel slippage at an optimal range for obtaining optimum vehicular braking efficiency. In general, optimum braking efficiency can be obtained at about 10% to 20% of wheel slippage. Excessive wheel slippage causes skidding or locking of the wheel to lower braking efficiency. Therefore, it is typical in anti-skid brake control to reduce braking pressure in response to a wheel slippage increasing across a predetermined threshold which may be set in the optimal wheel slippage range. By reduction of the braking pressure, wheel speed is recovered to lower wheel slippage across the threshold. Recovery of the wheel speed generally occurs due to an inertial moment on a vehicular body and frictional reacting force from the road surface.
In the prior proposed anti-skid brake control systems, recovery of wheel speed simply depends on road friction. Therefore, the speed of recovery of wheel speed varies significantly depending upon the road surface friction. Namely, on the high friction road, the frictional reacting force is greater to cause quick recovery of the wheel speed. Alternatively, when the road surface is very slippy, such as on the icy road, the frictional reacting force becomes very small to delay recovery of the wheel speed. This clearly increases braking distance.